I was running some errants today and was doing frequent stops. I pressed the start/stop button and the car started for few seconds then suddenly died. Engine Malfunction Power reduce came on. I tried to start the it again without luck. The engine kept trying to start for 5-6secs without any luck, I had to press the start/stop again to stop it. I let the car sit for 20secs and tried again. This time it started without any issues with SES on the dash. When I got home I read the ECU with the following error code -
U0100 Lost Communication With ECM/PCM "A". I didn't have any issues and the car ran just fine. What do you think might have cause it?
I do have JBD on default setting.

Those faults are hard to find. Could be a bad connection or corrosion on CAN connections. It could also be a software bug. U0100 also is set if the application layer CRC is incorrect. Unfortunately, U0100 is a collector DTC for many different faults. Without subfault description it's a needle in a haystack. Try to read "is memory" in ediabas.